You are given an isolated muscle with its motor nerve intact. This preparation is arranged so that you can make electrophysiological measurements from the nerve and the muscle as well as mechanical measurements of the muscle contraction. You observe that if caffeine is added to the fluid bathing the preparation, the peak tension developed in a muscle twitch produced by stimulating the motor nerve is increased.

A. Caffeine is known to alter transmembrane movement of Ca++. What are the possible sites at which caffeine is acting?

B. High concentrations of caffeine causes an isolated muscle to contract in the absence of any activity in the motor nerve. This contraction is not blocked by nicotinic blocking agents like curare. Where do you think caffeine acts?